conf: csd.preferred can now be set to ‘none’

When csd.preferred == none, we will request CSDs from the compositor,
but internally render as if we are using SSDs. That is, we don’t
render any window decorations at all.

Note that some compositors may ignore our request to use CSDs, and
still render SSDs for us.

Closes #163

config.h

@@ -140,7 +140,7 @@ struct config {
     } bindings;
 
     struct {
-        enum { CONF_CSD_PREFER_SERVER, CONF_CSD_PREFER_CLIENT } preferred;
+        enum { CONF_CSD_PREFER_NONE, CONF_CSD_PREFER_SERVER, CONF_CSD_PREFER_CLIENT } preferred;
 
         int title_height;
         int border_width;